It is often desirable to provide an auxiliary axle suspension for a road vehicle such as a truck-tractor or trailer to increase the load-carrying capacity of the vehicle. The present invention provides a novel spring lifting mechanism to enable the axle to be lifted when there is reduced load on the vehicle, thus saving both tire wear and engine fuel and enhancing vehicle maneuverability.
In the conventional pivoted auxiliary axle suspension, the load is transmitted to the vehicle frame by means such as one or more air bellows units. The axle lifting mechanism comprises one or more mechanical springs. When air is relieved from the air bellows, the mechanical springs pull the axle up to an elevated position out of ground engagement, the stress in the springs diminishing. When the air bellows are again pressurized, the wheels are forced downwardly into ground engagement with the stress in the mechanical springs increasing. This increase in mechanical spring stress is undesirable because it preloads the suspension and thus reduces the permissible load that the suspension can carry.
In the specific embodiment about to be described, the lifting mechanism comprises a cylinder having a reciprocable member therein that is operatively connected to the axle. A spring urges the reciprocable member in a direction to lift the axle. The cylinder is pivoted to the vehicle frame to enable the line of the lifting force to follow generally the movement of the axle as it is raised and lowered. This construction eliminates the linkage involved in some prior art constructions such, for example, as that shown in U.S. Pat. No. 3,771,812.
Further in accordance with the present invention, the pivot for the spring lifting mechanism is so located with respect to the pivot for the auxiliary axle suspension that the lever arm of the spring force line increases as the axle rises. Hence, the axle may be held elevated with less spring force. Moreover, as the axle descends and increases the spring stress, the lever arm of the spring force line decreases so that the lifting torque applied to the axle when its wheels are in ground engagement is reduced, similarly reducing the sacrifice in pay load.
Other objects of the invention will become apparent as the following description proceeds, especially when considered with the accompanying drawings.